1. Field of the Invention
The present invention relates to an optical recording medium in which information is recorded, reproduced and erased by a light such as a laser beam. More specifically, the present invention relates to an optical recording medium having a metal reflecting layer, and is particularly preferably applicable to a magneto-optical recording type optical recording medium.
2. Description of the Related Art
Optical recording media are now being investigated and developed because of their ability to provide a high density recording and their large capacity for storing information. Particularly, a magneto-optical recording medium is required, and various materials and systems have been proposed because of the wide applicability of these media to various fields.
Many magneto-optical recording materials including, for example, TbFe disclosed in Japanese unexamined Patent Publication (Kokai) No. 52-31703, TbFeGd disclosed in Japanese Unexamined Patent Publication (Kokai) No. 56-126907, TbFeCo and DyFeCo disclosed in Japanese Unexamined Patent Publication (Kokai) No. 58-73746, and NdFe disclosed in Japanese Unexamined Patent Publication (Kokai) No. 61-165846, have been proposed. Nevertheless, a further improvement of the recording and reproducing characteristics of the media using these materials must be made before these media can be practically used.
Accordingly, a metal reflecting layer formed on a magneto-optical recording layer or that formed on a dielectric layer present on a magneto-optical recording layer has been proposed, and this is advantageous because a high carrier to noise ratio (C/N ratio) is obtained by utilizing the Faraday effect as well as the Kerr effect. As such a metal reflecting layer, proposals including, for example, a layer of Al disclosed in Japanese Unexamined Patent Publication (Kokai) Nos. 58-83346 and 59-132434, a layer of Cu disclosed in Japanese Unexamined Patent Publication (Kokai) No. 59-8150, a layer of an Al-based alloy disclosed in Japanese Unexamined Patent Publication (Kokai) No. 62-137743, a layer of stainless steel disclosed in Japanese Unexamined Patent Publication (Kokai) No. 59-171054, a layer of Te disclosed in Japanese Unexamined Patent Publication (Kokai) No. 62-52744, and a layer of an amorphous metal disclosed in Japanese Unexamined Patent Publication (Kokai) No. 59-171054, have been made. Nevertheless, when a layer of a metal having a high reflectivity such as Ag, Al, Cu or the like is used, the recording sensitivity is remarkably lowered because of a high thermal conductivity thereof, and when a layer of a metal having a relatively low thermal conductivity such as Te, stainless steel or the like is used, the recording sensitivity is increased but a sufficiently high C/N ratio is not obtained because of a low reflectivity thereof.
Moreover, a recording layer tends to deteriorate under high temperature and high humidity atmospheric conditions, and therefore, the recording layer must be protected by the metal layer.
To solve the above problems, an addition of Ta to Al is proposed in Japanese Unexamined Patent Publication (Kokai) No. 64-4938, an addition of Ti, Mg or a transition metal to Au, Ag, Al or Cu is proposed in Japanese Unexamined Patent Publication (Kokai) No. 59-38781, an addition of Cu-Mg, Mg-Si, Cr, Sn, or Mg to Al is proposed in Japanese Unexamined Patent Publication (Kokai) No. 62-239349, and an addition of Ti to Al is proposed in Japanese Unexamined Patent Publication (Kokai) Nos. 62-137743 and 64-66847. These alloy layers bring an improvement of the thermal conductivity to some extent, while maintaining a high reflectivity thereof. Nevertheless, a considerable amount of the additive metal(s) is needed in the above alloys, to improve the durability of the medium under high temperature and high humidity conditions, and thus lowers the reflectivity below the level necessary for a reflecting layer. Accordingly, the above alloy reflecting layers cannot satisfy the requirements for both the reflecting layer and the protecting layer.
Furthermore, in the case of an Al alloy, a problem arises in that a uniform target of the alloy for sputtering is difficult to obtain, but such a uniform target is essential for commercially producing media using the Al alloy reflecting layer. Namely, a large difference of the melting points between Al and the additive metal prevents the production of a uniform Al alloy by a melting process. Namely, the melting point of Al is 660.degree. C. and those of the above additives are from 1453.degree. C. of Ni to 2977.degree. C. of Ta. Therefore, only a small amount of the additive metal can be uniformly mixed in the Al alloy, or if a larger amount of the additive metal is mixed for an effective lowering of the thermal conductivity of the alloy, only a non-uniform can be obtained.